In the electrical energy industry the various quantitative measurements which determine customer billing are traditionally obtained from a variety of different devices. Each device is designed and constructed to meter a particular quantity. For example, the historic Ferraris rotating meter is used in a variety of forms to measure real energy or reactive energy (KVAR) and with uniquely designed attachments, to measure related power demand quantities. KW and KVAR transducers when coupled with appropriate integrating means are also used for these purposes. Inherent in the electromechanical structure or electronic circuitry of each such device are the mathematical operations which are necessary to determine the particular quantity.
For example, the rotating Ferraris Kwh meter inherently provides a cumulative total number of output revolutions which is proportional to the product of current, voltage, the cosine of the angle between them and time in hours. Similarly, a KVAR transducer provides an output which is proportional to the product of voltage, current and the sine of the angle between them.
With conventional technology a power company orders and installs those devices or transducers which provide the quantities which are desired. Some manufacturers have combined more than one type of transducer into an aggregation of such devices in a single piece of equipment. U.S. Pat. No. 4,218,737 shows two different transducers combined in a circuit which converts each of their analog outputs to digital form and then processes, stores, communicates and displays the processed data.
Prior art analog/digital conversion techniques include the sampling of a periodic signal by generating a ramp function and continuously comparing the ramp to the sample until there is a transition of the output state of a comparator when the amplitude of the ramp intersects the amplitude of the sample. If the ramp is generated by a digital counter which is periodically incremented and drives a D/A converter, then the accumulated count of the counter is a digital measurement of the amplitude of the sample.
However, if these prior art techniques were applied to the sampling of two or three voltage and two or three current signals from a three phase electrical energy distribution system, a total of four to six of these conventional analog/digital converter circuits would be required. Not only would this require excessive duplication of circuitry, but there would be significant problems in both the synchronization of the circuits so that the samples could be taken simultaneously or at accurately selected times and in the standardization of the gain of the circuitry so that all samples would be accurate at all times.
Another difficult problem with prior art analog to digital converters would be the microprocessor time that would be required for control of four to six such converters. The long time requirement for the sampling would also result in inaccuracies due to signal changes which would occur between samples.
Conventional sampling techniques also call for a selected number of samples taken at specific intervals across a cycle. For example, samples might be taken at 45.degree. intervals. However, the presence of higher order harmonics and transient conditions allow some inaccuracies to occur with such conventional sampling technique because of the unsampled intervals between the regularly spaced sample positions.